Allow overriding log handler in hs.logger. Add option to log to file.

[gineer01]

PR for #1684

Allow overriding log handler in hs.logger with hs.logger.logHandler().
User can use hs.logger.createFileLogHandler(path) to create a log handler that logs to a file at the provided path.
Refactor the current handling to the constant logger.defaultLogHandler and use it as the default value for logHandler.

Some example code (also in the function documentation):

local fileHandler = hs.logger.createFileLogHandler('path/example.log')
hs.logger.logHandler(function(log)
   fileHandler(log)
   hs.logger.defaultLogHandler(log)
end)

[stickler-ci]

(W614) trailing whitespace in a comment

[codecov]

Codecov Report

Merging #2627 (c924260) into master (5ba3efa) will decrease coverage by 19.60%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #2627       +/-   ##
==========================================
- Coverage   27.07%   7.47%   -19.61%     
==========================================
  Files         177       3      -174     
  Lines       35301    1258    -34043     
==========================================
- Hits         9557      94     -9463     
+ Misses      25744    1164    -24580     

[muescha]

should names be ConsoleLogHandler and FileLogHandler?

or better as objects ConsoleLogger and FileLogger where we have :init and :log method?

[cmsj]

Since this has gotten stale and has a merge conflict, I'm going to close it for now. Please feel free to re-open (or open a new PR) if you're able to resolve the conflicts.